Conventional motor drive control devices include one that is configured to generate a direct-current bus voltage for driving an inverter from an alternating-current power supply, such as a commercial power supply. Such a motor drive control device is used, for example, as a drive source for driving a motor of a compressor of an air-conditioning apparatus.
When the motor drive control device is used as a drive source for driving a motor of a compressor of an air-conditioning apparatus, the motor is often designed, for the purpose of improving energy consumption efficiency during rated operation, so that an induced voltage (counter electromotive force) and a power supply voltage are substantially the same at around a rated rotation speed.
For example, in an overload operation where the motor is operated at a rotation speed exceeding its rated rotation speed, the output current is increased by saturation of the output voltage of an inverter circuit. As a result, in the overload operation different from the rated operation, the operation efficiency of the motor or the inverter circuit is decreased.
To reduce the decrease in the operation efficiency during the overload operation, a motor drive control device including a boosting circuit is proposed (see, for example, Patent Literature 1).
The boosting circuit is disposed between a rectifying circuit and an inverter circuit, and includes a reactor, a backflow preventing diode, and a switching element. A direct-current bus voltage rectified by the rectifying circuit is boosted by the boosting circuit. The boosting circuit accumulates energy in the reactor while the switching element is on. While the switching element is off, the boosting circuit releases the energy accumulated in the reactor, thus boosting the direct-current bus voltage.
The boosting operation performed by the boosting circuit to boost the direct-current bus voltage is controlled by a time period during which the switching element is on, that is, by the on-duty ratio of the switching element. Because the boosting operation performed by the boosting circuit to boost the direct-current bus voltage increases a voltage applied to the motor, a current flowing through the motor is suppressed. Thus, the conventional motor drive control device improves the operation efficiency by suppressing the current flowing through the motor as a result of the boosting operation of the boosting circuit, and increases the operating range by increasing the voltage applied to the motor.
In the conventional motor drive control device, when the boosting circuit performs a boosting operation, driving the switching element causes a circuit loss. Thus, in the conventional motor drive control device, when the boosting circuit performs a boosting operation, the circuit loss of the switching element may cause decrease in operation efficiency. Thus, the conventional motor drive control device restricts the boosting operation of the boosting circuit within the operating range where the voltage needs to be boosted.
Conventional motor drive control devices also include one that is configured to detect the pulsation amplitude (ΔV) of the direct-current bus voltage and perform, if the detected pulsation amplitude exceeds a preset value, protective control for suppressing the output power of an inverter, thereby suppressing a ripple current flowing through a smoothing capacitor (see, for example, Patent Literature 2).